Auxiliary Memory in Computer Architecture

Auxiliary Memory in Computer Architecture – Auxiliary memory is the non-volatile memory with the lowest cost, highest capacity, and slowest access in a computer system. It is where programs and data are kept for long-term storage or when not in immediate use. It normally stores system programs, instruction and data files. It is also known as secondary memory.

It can also be used as an overflow/virtual memory in case the main memory capacity has been exceeded. Secondary memories cannot be accessed directly by a processor. First, the data/information of auxiliary memory is transferred to the main memory, and then that information can be accessed by the CPU.

The most common examples of auxiliary memories are:

• Magnetic disks
• Magnetic tape

Magnetic Disks

A magnetic disk is a type of memory constructed using a circular plate of metal or plastic coated with magnetized materials. Usually, both sides of the disks are used to carry out read/write operations. However, several disks may be stacked on one spindle with a read/write head available on each surface. The diagram below shows the structural representation of a magnetic disk.

• The memory bits are stored in the magnetized surface in spots along the concentric circles called tracks.
• The concentric circles (tracks) are commonly divided into sections called sectors.

There are two types:

1. Hard disks are made of rigid aluminum or glass.
2. Removable diskettes made of flexible plastic.

In 1956, the first magnetic hard drive (HD) was invented at IBM, consisting of 50 21-inch (53-cm) disks; it had a storage capacity of 5 megabytes. By the 1990s, the standard HD diameter for PCs had shrunk to 3.5 inches (about 8.9 cm), with storage capacities in excess of 100 gigabytes (billions of bytes); the standard size HD for portable PCs (“laptops”) was 2.5 inches (about 6.4 cm).

Since the invention of the floppy disk drive (FDD) at IBM by Alan Shugart in 1967, diskettes have shrunk from 8 inches (about 20 cm) to the current standard of 3.5 inches (about 8.9 cm). FDDs have low capacity, generally less than two megabytes, and have become obsolete since the introduction of optical dise drives in the 1990s.

Magnetic Tape

Magnetic tape is a storage medium that allows data archiving, collection, and backup for different kinds of data. The magnetic tape is constructed using a plastic strip coated with a magnetic recording medium. Tape is cheap, but access time is far slower than that of a magnetic disk because it is sequential-access memory-i.e., data must be sequentially read and written as a tape is unwound, rather than retrieved directly from the desired point on the tape. Servers may also use large collections of tapes or optical discs, with robotic devices to select and load them, rather like old-fashioned jukeboxes.

The hits are recorded as magnetic spots on the tape along several tracks. Usually, seven or nine bits are recorded simultaneously to form a character together with a parity bit.

Magnetic tape units can be halted, started to move forward or in reverse, or can be rewound. However, they cannot be started or stopped fast enough between individual characters. For this reason, information is recorded in blocks referred to as records.

Characteristics of Auxiliary Memory are following?

• Non-volatile memory – Data is not lost when power is cut off.
• Reusable – The data stays in the secondary storage on permanent basis until it is not overwritten or deleted by the user.
• Reliable – Data in secondary storage is safe because of high physical stability of secondary storage device.
• Convenience – With the help of a computer software, authorised people can locate and access the data quickly.
• Capacity – Secondary storage can store large volumes of data in sets of multiple disks.
• Cost – It is much lesser expensive to store data on a tape or disk than in primary memory.